1. Field of the Invention
This invention relates generally to wireless data communication, and, in particular, to transmit power control by which an optimal transmitter power is determined that is high enough to enable reliable communication while low enough to minimize interference to other users or devices sharing the same spectrum.
2. Description of the Related Art
In wireless data communication systems, it is often beneficial to employ transmit power control (TPC), limiting the transmit power to a level high enough for reliable communication but typically less than the maximum available power. Benefits include but are not limited to reduced transmitter power drain—especially important in battery-powered applications—and reduced interference to other users of the same spectrum. In some systems such as ultra wideband (UWB), concurrent users share all or a portion of the spectrum used by other users' transmissions.
TPC is widely used in cellular telephone systems and wireless data communication systems utilizing unlicensed spectrum, such as that system commonly referred to as Wi-Fi. In communication systems utilizing spread-spectrum modulation, minimizing the transmit power is especially important, as multiple transceivers in an area share common spectrum. The effectiveness of communication between devices may be reduced considerably if one or more transmitters in the area are transmitting at significantly higher power than the other transmitters. TPC is typically implemented as an iterative process converging on an optimal transmit power, wherein a first transceiver transmits a first data packet at a high level, typically maximum power. If a second transceiver is within range, it receives this transmission and computes a figure of merit, such as frame error rate (FER), which is related to received signal power. This figure of merit is compared in the second data transceiver to desired limits, and a command to increase power or decrease power is transmitted back to the first data transceiver. The first data transceiver then typically raises or lowers power in a stepwise manner, or according to another power level progression. Another data packet is then sent to the second transceiver, using this modified transmit power, and a new figure of merit is computed and compared to desired limits, causing another increase or decrease power command to be sent to the first transceiver. In this iterative manner, a transmit power level for the first transceiver is found which generates the desired figure of merit in the second transceiver. Measurement of the figure of merit may continue as the payload data transfer occurs, so that the iterative process of adjusting transmit power may be repeated if the figure of merit deviates from the prescribed range. In typical systems, the transmit power levels in both transceivers are adjusted in this manner, either concurrently or sequentially.
The iterative process described above typically requires multiple bidirectional data exchanges to arrive at optimal transmit power levels for each transceiver. These data exchanges add undesired overhead to the communication link, putting additional drain on the power source in each transceiver, and lengthening the time each transmitter is on the air and thus potentially interfering with other transceivers in the area. A method and apparatus for optimizing this process so as to more rapidly determine an optimal transmit power is therefore desirable.